6J6
The 6J6 (European: ECC91) (P/N 154-0032-00) is a high-frequency dual triode vacuum tube with a 6.3 V, 0.45 A heater.
Introduced in 1939/1940, it was one of the earliest tubes with a miniature 7-pin B7G base. It is used in the 512 and possibly other instruments.